While using cr_cn, I realized that one doi works with link-based content type requests
curl http://api.crossref.org/works/10.5604/20831862.1134311/transform/application/vnd.crossref.unixsd+xml
but not with accept header statements
curl -LH "Accept: application/vnd.crossref.unixsd+xml http://dx.doi.org/10.5604/20831862.1134311
In order to avoid confusion in future, I thought it might be helpful, if we switch to link-based content type requests not only for citeproc-json (see #62), but for all formats except text and datacite-xml